1. Field of the Invention
The present invention relates, in general, to stepping motors for optical pick-up devices and, more particularly, to a structural improvement in such stepping motors to simplify the support structure for a motor's rotating shaft engaging with a pick-up unit at a lead screw and feeding the pick-up unit by the screw in an axial direction, the structural improvement thus reducing the number of required elements, simplifying the assembling process and improving the operational reliability and productivity of stepping motors.
2. Description of the Prior Art
As well known to those skilled in the art, a stepping motor, also known as a step-servo motor, is provided within an optical pick-up device for feeding a pick-up unit. In such a stepping motor, a rotating shaft has a lead screw and engages with the pick-up unit at the lead screw, thus allowing the pick-up unit to reciprocate in an axial direction along the lead screw in accordance with a rotating action of the rotating shaft.
An example of conventional stepping motors for optical pick-up devices may be referred to Japanese Patent Laid-open Publication No. Heisei. 9-154,217. The above Japanese stepping motor is described hereinbelow with reference to FIGS. 1 and 2.
As shown in FIG. 1, the conventional stepping motor comprises a stator 100 and a rotating shaft 200. The stator 100 consists of a casing 110, two doughnut coils 120 and a yoke 130.
In the stator 100, the casing 110 has a hollow cylindrical shape, with the top and bottom walls radially and inwardly extending from the top and bottom circular ends of the casing's outer wall so as to form an annular shape. The top and bottom walls of the casing 100 also respectively extend from their inside ends toward each other in a vertical direction. That is, the top wall extends downwardly from its inside end to form an upper inner wall, while the bottom wall extends upwardly from its inside end to form a lower inner wall, with a gap being left between the ends of the two inner walls.
The yoke 130, providing a passage for magnetic flux within the stator 100, is horizontally positioned within the gap between the ends of the two inner walls of the casing 110 and is integrated with the interior surface of the outer wall of the casing 110. The yoke 130 thus forms two annular chambers within the casing 110, with the two doughnut coils 120 being respectively received within the two chambers.
The above stator 100 is fixedly supported on a first support plate 600 of an optical pick-up device at its bottom wall.
A rotating shaft 200 passes through the center of the first support plate 600 upwardly prior to being partially inserted into the center of the stator 100 in a direction from the bottom of the casing 110. The above shaft 200 is externally threaded at a portion exposed to the outside of the support plate 600, thus forming a lead screw 210 which movably engages with a pick-up unit.
A cylindrical magnet 220 is fixedly fitted over the top end portion of the rotating shaft 200 within the casing 110 of the stator 100, and so the magnet 220 and the doughnut coils 120 are concentrically positioned within the casing 110. When the coils 120 are turned on, an electromagnetic force is formed between the magnet 220 and the coils 120 within the casing 110.
Both ends of the shaft 200 are each provided with a V-shaped groove 230, with a steel ball 240 being rotatably seated on each groove 230.
A guide plate 300 is positioned on the top wall of the stator 100. Vertically formed at the central portion of the above plate 300 is a guide hole having a predetermined diameter.
A first holder 410, or a movable holder having a depressed ball seat on its lower surface, is received within the guide hole of the guide plate 300 in a way such that the holder 410 is vertical movable within the guide hole of the plate 300 with the depressed seat being directed downwardly. A first steel ball 240 is rotatably seated between the depressed ball seat of the first holder 410 and the top groove 230 of the rotating shaft 200.
The above guide plate 300, positioned on the top wall of the stator 100, is covered with a cap 500 that is fitted over the top end portion of the stator 100. The top wall of the cap 500 is partially cut along a U-shaped cut line at one or more angularly spaced positions as shown in FIG. 2, thus forming one or more cut pieces. The cut pieces of the cap 500 are, thereafter, bent downwardly at an angle of inclination, thus forming one or more plate springs 510.
When the cap 500 is fitted over the top end portion of the stator 100, each plate spring 510 is brought into contact with the top surface of the first holder 410 at its free end, thus normally biasing the first holder 410 downwardly.
The skirt of the cap 500 comes into elastic engagement with the external surface of the casing 110 at its lower edge, and so the cap 500 is easily removable from the stator 100 when necessary.
A second steel ball 240, rotatably seated on the bottom groove 230 of the rotating shaft 200, is also rotatably seated in the depressed ball seat of a second holder 420. That is, the second steel ball 240 is rotatably seated between the bottom groove 230 of the shaft 200 and the ball seat of the second holder 420. The above second holder 420 is fixedly mounted to a second support plate 700, thus being so-called "a fixed holder".
It is thus noted that the rotating shaft 200 is elastically supported by the first holder 410 rather than the second holder 420.
When the coils 120 of the stator 100 are activated by electric power from an external power source, an electromagnetic force is formed between the magnet 220 and the coils 120, thus rotating the shaft 200. When the rotating shaft 200 is rotated as described above, the pick-up unit, engaging with the lead screw 210 of the shaft 200, axially moves along the lead screw 210.
In the above-mentioned stepping motor, the rotating shaft 200 is designed to be rotated in opposite directions so as to allow the pick-up unit to axially reciprocate along the lead screw 210 as desired. The shaft 200 thus may undesirably move in the axial direction due to an inertia force, that is generated from the pick-up unit at a time the moving direction of the pick-up unit is changed. If such an undesirable axial movement of the rotating shaft 200 is not effectively absorbed by a shock absorption means, the shaft 200 may be impacted, damaged and deformed.
In an effort to absorb impact caused by such an undesirable axial movement of the rotating shaft 200 due to the inertia force, the conventional stepping motor is provided with the guide plate 300, the first holder 410 and the plate springs 510.
In a detailed description, the second steel ball 240 is rotatably seated between the bottom groove 230 of the rotating shaft 200 and the ball seat of the fixed second holder 420, thus only rotatably supporting the bottom end of the shaft 200. Meanwhile, the first steel ball 240 is rotatably seated between the top groove 230 of the rotating shaft 200 and the depressed ball seat of the first holder 410 which is normally biased downwardly by the plate springs 510 of the cap 500, thus absorbing the impact caused by the undesirable axial movement of the shaft 200.
Since the undesirable axial movement of the rotating shaft 200, caused by the inertia force generated from the pick-up unit at a time the moving direction of the pick-up unit is changed, is absorbed by the plate springs 510 of the cap 500 elastically biasing the first holder 410 downwardly, the rotating shaft 200 is somewhat stably operated even when the axial moving direction of the pick-up unit is changed.
However, the above-mentioned stepping motor is problematic in that it has a complex construction.
That is, a V-shaped groove 230 is necessarily formed on each end of the rotating shaft so as to rotatably support the reversibly rotatable shaft 200 while absorbing an undesirable axial movement of the shaft 200 due to an inertia force. In addition, one steel ball 240 has to be rotatably seated on each groove 230, while two holders 410 and 420 rotatably support the two steel balls 240 at both ends of the shaft 200.
Furthermore, it is necessary to elastically support the first holder 410 by the plate springs 510 of the cap 500 while allowing the holder 410 to be vertically movable along with the top end of the shaft 200 so as to absorb an undesirable axial movement of the shaft 200 when the moving direction of the pick-up unit is changed.
Such a plurality of elements for supporting the rotating shaft 200 increase the production cost, complicate the production process, and reduce productivity while producing the stepping motors. The elements are also assembled together while being brought into frictional engagement with each other. The elements thus generate operational noises while being frictionally abraded, and so the stepping motor is inconvenient to users and is reduced in durability and operational reliability.
Another disadvantage of the conventional stepping motor resides in that it is necessary to precisely machine the V-shaped grooves 230 on both ends of the rotating shaft 200. The V-shaped grooves 230 thus make the process of producing the stepping motors very difficult.